Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. A majority of the refinery gasoline blending pool in the United States is produced by this process, with almost all being produced using the fluid catalytic cracking (FCC) process. In the FCC process, heavy hydrocarbon fractions are converted into lighter products by reactions taking place at high temperatures in the presence of a catalyst, with the majority of the conversion or cracking occurring in the gas phase. The FCC hydrocarbon feedstock (feedstock) is thereby converted into gasoline and other liquid cracking products as well as lighter gaseous cracking products of four or fewer carbon atoms per molecule. These products, liquid and gas, consist of saturated and unsaturated hydrocarbons.
In FCC processes, feedstock is injected into the riser section of a FCC reactor, where the feedstock is cracked into lighter, more valuable products upon contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator. As the endothermic cracking reactions take place, carbon is deposited onto the catalyst. This carbon, known as coke, reduces the activity of the catalyst and the catalyst must be regenerated to revive its activity. The catalyst and hydrocarbon vapors are carried up the riser to the disengagement section of the FCC reactor, where they are separated. Subsequently, the catalyst flows into a stripping section, where the hydrocarbon vapors entrained with the catalyst are stripped by steam injection. Following removal of occluded hydrocarbons from the spent cracking catalyst, the stripped catalyst flows through a spent catalyst standpipe and into a catalyst regenerator.
Typically, catalyst is regenerated by introducing air into the regenerator and burning off the coke to restore catalyst activity. These coke combustion reactions are highly exothermic and as a result, heat the catalyst. The hot, reactivated catalyst flows through the regenerated catalyst standpipe back to the riser to complete the catalyst cycle. The coke combustion exhaust gas stream rises to the top of the regenerator and leaves the regenerator through the regenerator flue. The exhaust gas generally contains nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), oxygen (O2), ammonia, nitrogen and carbon dioxide (CO2).
The three characteristic steps of the FCC process that the cracking catalyst undergoes can therefore be distinguished: 1) a cracking step in which feedstock is converted into lighter products, 2) a stripping step to remove hydrocarbons adsorbed on the catalyst, and 3) a regeneration step to burn off coke deposited on the catalyst. The regenerated catalyst is then reused in the cracking step.
The presence of metal contaminants in feedstock presents a serious problem. Common metal contaminants are iron (Fe), nickel (Ni), sodium (Na), and vanadium (V). Some of these metals may promote dehydrogenation reactions during the cracking sequence and result in increased amounts of coke and light gases at the expense of gasoline production. Some of these metals may also have a detrimental effect on the FCC of feedstock and cracking catalyst stability and crystallinity.
During the cracking catalyst regeneration process, metals present in the catalyst itself may volatilize under the hydrothermal conditions and re-deposit on the catalyst. Silicon (Si) is an example of such a metal.
All of these metals, whether initially present in the feedstock, the cracking catalyst, or some other compound present in the FCC reactor, may lead to loss of activity, selectivity, stability, and crystallinity of the active component of the cracking catalyst.
Vanadium poisons the cracking catalyst and reduces its activity. The literature in this field has reported that the V compounds present in feedstock become incorporated in the coke which is deposited on the cracking catalyst and is then oxidized to vanadium pentoxide in the regenerator as the coke is burned off. One possible pathway by which V reduces catalyst activity involves vanadium pentoxide reacting with water vapor present in the regenerator to form vanadic acid. Vanadic acid may then react with the zeolite catalyst, destroying its crystallinity and reducing its activity.
Because compounds containing V and other metals cannot, in general, be readily removed from the cracking unit as volatile compounds, the usual approach has been to passivate these compounds under conditions encountered during the cracking process. Passivation may involve incorporating additives into the cracking catalyst or adding separate additive particles along with the cracking catalyst. These additives combine with the metals and therefore act as “traps” or “sinks” so that the active component of the cracking catalyst is protected. These metal contaminants are removed along with the catalyst withdrawn from the system during its normal operation and fresh metal trap is added with makeup catalyst so as to effect a continuous withdrawal of the detrimental metal contaminants during operation. Depending upon the level of the harmful metals in the feedstock, the quantity of additive may be varied relative to the makeup catalyst in order to achieve the desired degree of metals passivation.
Industrial facilities are continuously trying to find new and improved methods to increase the performance of cracking catalysts. The present invention is directed to these and other important ends.